Strains of E. coli from diverse natural sources are being studied with respect to allozyme variation, and the natural genetic markers thus revealed are being used to investigate recombination and infectivity under quasi-natural (laboratory-animal) conditions. An attempt to demonstrate neutral polymorphism in the DNA base sequence is planned. Drosphila melanogaster cage populations have been followed with respect to genotype frequencies and linkage disequilibrium involving three linked enzyme loci. Derivative cages, each started by a single female, are now under study. Also the phenomenon of population control by emigration has proved accessible to study in cage populations. Possible variation in heat stability of alcohol dehydrogenase and other enzymes in Drosophila and E. coli is being investigated. Allozyme polymorphism in laboratory balanced lethal stocks is being surveyed as a result of the finding that in one such stock, one chromosomal arrangement is dimorphic at the alcohol dehydrogenase locus.